Light and dark have significant direct effects on sleep, temperature, and neuroendocrine function independent of their roles as regulators of circadian rhythms. Light therapy is being used with increasing frequency to treat a variety of conditions, including depression, circadian rhythm disorders, and sleep disturbances related to aging. Although the mechanisms for light entrainment of circadian rhythms have been studied extensively, the mechanisms for direct photic effects on sleep and temperature have not been elucidated. A better understanding of the direct effects could make light a more powerful clinical tool. We propose to study the mechanisms for direct light effects on sleep in an animal model. Light promotes sleep in both albino and pigmented rats. However, our research has shown that the effect on rapid eye-movement sleep (REM) is opposite in albino and pigmented rats. Light suppresses REM in albinos but stimulates it in pigmented rats. Albino rat show REM triggering following lights-off, whereas pigmented rats show decreases in REM during dark periods. Differences in central visual pathways have been described in albino vs. pigmented mammals. It is likely that albino and pigmented rats show differences in REM patterns in response to light because of differences in their visual systems. The existence of both behavioral and anatomic differences associated with pigmentation status provides a model for studying the mechanisms mediating the direct effects of light on sleep. The following studies are proposed to determine direct effects of light and dark on sleep induction, REM regulation, and temperature in congenic albino and pigmented rats: 1. Determination of how the direct effects of light vary with light intensity, wavelength, and duration. 2. Determination of the effect of age on the direct responses to light. 3. Investigation of the mediation of the direct effects by comparing visual pathways in albino and pigmented rats, lesioning specific visual pathways, and studying mutant strains of rats with retinal hypopigmentation.